Cooling towers are industrial cooling units operating to dissipate heat. As any surface in contact with aqueous systems, biofilm formation appears on the surface of heat exchangers. Although the biofilm formation on plastic tower fill in wet cooling towers has been studied widely, no studies found regarding biofilm formation on the steel heat exchangers of the closed-loop. Heat exchanger were coated with nano-silica, where known to reduce the formation of biofilm. Natural biofilm formation was monitored for 6 months. Biofouling was examined monthly using epifluorescence microscopy by assessing the numbers of live/dead bacteria. It was observed that the biofilm layer formed on the nano-silica coated heat exchanger surfaces was significantly lower than the control surfaces. 3 log microbial reduction was recorded on coated surfaces in the first month. After 6 months, total biomass on control coupons were reached to 1.28 x 1012 cell/cm2, where the biomass on nano-silica coated coupons was 6.3 x 104.